Controller having its response modified by motor position



CONTROLLER HAVING ITS RESPONSE MODIFIED BY MOTOR POSITION Filed Oct.v 4,195o April 20, 1954 wQ R. KEHOE 2 Sheeis-Sheet l Hran M54 s :mini/vrTeA/vsnl rrER l'mventor Gttorneg April 20, 1954 w. R. KEHOE 2,675,826

CONTROLLER HAVING ITS RESPONSE MODIFIED BY MOTOR POSITION 2 Sheets-Sheet2 Filed OG'C. 4. 1950 nventor ma@ KM lQu/m'inkomeg Patented Apr. 20,v1954 CONTROLLER HAVING ITs RESPONSE MODIFIED BY MOTOR POSITION WilliamR. Kehoe, Erie, Pa., assi'gnor to American Meter Company, Erie, Pa., acorporation of Delaware Application October 4, 1950, lSerial No. 188,340

Claims.

in fluid operated controllers where a control pressure is produced inresponse to the deviation oi a controlled quantity from the operatingpoint, there is a tendency to hunting which has been corrected bythrottling and reset means modi tying the control pressure produced bythe controller.

When the control pressure is used to operate a motor to vary thecontrolled quantity, there isv a plienomenom in the nature of hysteresiswhich inhibits accurate positioning of the motor. Motor hysteresis hasbeen overcome by so-called positioners or servo mechanisms fed by thecontrol pressure.

This invention is intended to eliminate the positie-ner for the motorand the throttling and reset means in the controller and to achieve thesame or better end result by having the thrcttling and reset meansoperated by the motor position. By this simpler arrangement there lisintroduced a temporary compensation, change in motor position,corresponding to the difference between the measured value of thequantity and the desired or set point of the value ci' the quantity andthis compensation is graduaily reduced to zero as the measured quantityapproaches the position corresponding to the set point. By having thecompensation respond directly to the motor position, the hysteresis iseliminated and the motor can be more accurately positioned with respectto the control impulse. FrorA one aspect the throttling and reset (withor without derivative response) is transferred from the motor operatingcontrol pressure to the motor position, This has particular advantagesin systems 'where the chart room is remote from the motor and controlledand in which the controiler information is transmitted to the chart roomby transmitters. Further objects and advantages appear in thespecication and claims.

1n the drawing, Fig. l is a diagrammatic view oi a control; Fig. 2 is' asection through a less elaborate relay for use 'in the Fig. l control;and Fig. 3 is a section through a modification'.

leierring to the drawing, l indicates a sensitive measuring element'for' the controlled quantity and having a shaft' 2 carrying a pen arm 3for a chart fi. At the same radial distance from the center oi the chartas the measuring element is a iixed pivot 5 carrying a bell crank resetlever t one arm 'i of which serves as a direct setA pointer indicatingthe desiredv value of the controlled quantity and the vother arm 8 ofwhich is pivoted to one end oi a diierential lever 9'. The opposite endof the differential lever is' connected' by a link lo to an arm il ofthe shaft 2. In the balanced position the direct set pointer I indicatesthe same value as the pen arm 3 and the differential lever is parallelto the arm I l and in line through the pivot l5 so the arm Il, link l0,and differential lever 9 for this position, namely the balancedposition, form a parallelogram linkage. The parts are illustrated in anexaggerated on balance position.

Instead of having a sensitive element i control the pen arm 3 it ispossible to have the position oi' the pen arm 3V controlled by a bellowsla connected to the arm I i on the shaft 2 and fed from a line ib from atransmitter associated with a remote sensitive element. The pressure inthe bellows la would at all times correspond to the indication oi thesensitive element and accordingly the position of the pen arm 3 wouldcorrespond to the measurement information of the controlled quantity astransmitted to the pen arm by the sensitive element transmitter. It isalso possible to have a bellows 6a fed from a set point transmitterthrough a line Sb and connected to the arm 8 of thebell crank lever 5 toposition the direct set pointer T in accordance with the desired or setpoint for the controlled quantity. The set point transmitter also couldbe located remote from the chart.

Whether the pen arm 3 and the direct set pointer l are positioned at thechart or by information obtained from transmitters remote from the chartis immaterial. In either case, there is a comparison between themeasured value of the controlled quantity and the set point or desiredvvalue of the controlled quantity which is utilized to operate thecontrol.

Changes in the value of the controlled quantity are indicated` byrotation of the shaft 2 andV of the pointer 3 and arm Il xed thereto.The movement of the arm li is transmitted to the diierential lever 9causing it to pivot about the lower end of the arm il. The motion of thedifferential lever is transmitted to a llapper lever l2 pivoted on apivot i3 and having one end pivoted at i4 to the differential lever e,and its other end carrying a dapper l5 cooperating with. a nozzle lesupplied from a pneumatic fluid pressure' line l'i through a restrictedorifice I8 and a passageway i9. The pressure in the passageway l 9varies directly with the position of the dapper, incr-easing as thedapper approaches and decreasing as the dapper recedes from the nozzle'.The orifice i8 so restricts the flow that the variations in pressure inthe passageway i9 do not materially vary the quantity of fluid flow.

The pressure in passageway I9 is fed through a way 2@ to the interior ofa bellows 2l carried by a rigid wall 22 and loaded by a compressionspring 23. The bellows 2| acts through a relatively light couplingspring 2s attached to the free end plate 25 of an enclosing bellows 255also carried by the rigid wall 22. The end plate 25 carries a valve 'lhaving oppositely facing conical faces 28 and 29 respectivelycooperating with a seat 3U in a Way 3l leading from the fluid pressuresupply Il and with a seat 32 on a bushing 33 loosely surrounding thevalve stem Sli. The valve 2? iioats between the valve seats 39 and 32and thereby controls the pressure in a line 35 leading from a chamber 3daround the valve seats to a duid motor 3l which operates a controlmember for applying an actuating force to the controlled quantitychanging the value of the controlled quantity, for example, a valve 3Mhaving a stem Sib connected to the diaphragm 31e of the fluid motor 3l.The fluid pressure delivered to the iuid motor 31, which correspends tothe diierence between the measured value of the controlled quantity andthe set point or desired value or the controlled quantity, isaccordingly utilized to change the setting of the control member for thecontrolled quantity and to thereby introduce a correction in thedirection to return the controlled quantity to the desired or set pointvalue.

Upon changes in pressure in the passageway 19, due to deviations in themeasured value of the controlled quantity from the desired or set pointvalue, the bellows 2l initially moves the valve 21 in the direction toproduce a change in the pressure in the line 35 in the direction tocause operation of the motor 37 to bring the controlled quantity back tothe desired value. For example, upon an increase in the magnitude of thecontrolled quantity, the motor 37 would actuw ate the valve stern 31h inthe direction to decrease the magnitude of the controlled quantity. Inother words, the bellows 2i acts initially to produce a change inpressure in the line 35 in the direction to compensate for deviations inthe measured value of the controlled quantity from the desired or setpoint value.

The bellows 26 which is larger and more sensitive than the bellows 2 i,is fed from the way 29 through a needle valve 33 which introduces anadjustable time lag between the response of the bellows 216 and theresponse of the bellows 2i. That is, the initial compensating action ofthe bellows 2l is followed up by a delayed compensating action of thebellows 26. The bellows or diaphragms 2| and 25 comprise a controlpressure relay. The sensitivity of the relay is adjustable by the needlevalve 3B which permits matching of the time lag or sensitivity of therelay to the system.

instead of the control pressure relay illustrated in Fig. 1 which isdescribed in greater detail in application Serial No. 108,775, filedAugust 5, 1949, it is possible to substitute in the Fig. 1 system theless elaborate control pressure relay illustrated in Fig. 2. The Fig. 2control pressure relay has a passageway ld supplied from the fluidpressure line Il through a restricted orice 18a, which supplies asubstantially constant fluid flow to the nozzle IS. The pressure at thenozzle Hi which appears in the passageway ld is fed through a way a tothe interior of a bellows 21a carried by a rigid wall 22a. The bellows21a has an end plate a which carries a valve 21a having oppositelyfacing conical faces 2da and '2da respectively cooperating with a seatSila in a way 3 la leading from the iluid pressure supply il and with aseat 32a on a bushing 33a loosely surrounding the valve stem Ma. r1Ehevalve 21a floats between the valves 30a and 32a and thereby controls thepressure in the line 35 leading from the chamber 36a around the valveseats to the fluid motor 3l'. The less elaborate control pressure relayillustrated in Fig. 2 likewise produces a control pressure in the line35 leading to the motor 3l, which corresponds to the relative positionof the nozzle and ilapper l5, it.

The function of the control pressure relays illustrated, or of the othercontrol pressure relays which could be substituted in the Fig. 1 system,is to produce a control pressure corresponding to the nozzle and flapperseparation.

Since the nozzle and flapper separation corresponds primarily to thedeviation of the measured value of the controlled quantity from thedesired or set point value, the control pressure produced by the controlpressure relay corresponds to the magnitude of the deviation of thecontrolled quantity from the desired or set point value. The mechanicallinkage illustrated in Fig. l produces a small variation in apperposition under steady state conditions due to the fact that thedifferent lever d is in line with the ixed pivot 5 and the motion of thedifferential lever transmitted to the iiapper is due to the angularmotion of the differential lever about the fixed pivot 5. When the pivotIA is close to the pivot 5, a large variation in the angularity of thediiferential lever Si results in a small variation in the ilapperposition.

While the lapper position is essentially, or almost constant understeady state conditions, there is a substantial variation in flapperposition under transient conditions. Movement of the pen arm 3 away fromthe position set by the direct set pointer l causes movement or" thedifferential lever il to one side of the fixed pivot 5 thusy producing amuch larger change in the nozzle pressure than the full range of steadystate nozzle pressures. This greater change in nozzle pressure can beexplained as due to the diiference between the pivoting of thedifferential lever t about the pivot 5 and about the lower end of thearm e; pivoting about the lower end oi' the arm 2 obviously results inthe greater change in Trapper position and resultant nozzle ressure.This means that a small movement of the pen arm 3 from the desiredposition indicated by the direct set pointer l results in a large'transient change in nozzle pressure.

Under steady state conditions, when the measured value of the controlledquantity indicated by the pen arm il corresponds exactly with theoperating point indicated by the direct set pointer l, there is no feedback; oi the control pressure from the motor 37 to the flapper l5. Underthese conditions the pressure in the motor El' corresponds precisely tothe nozzle and flapper separation which is determined by the operatingpoint.

However, under transient conditions, when the nozzle and flapperseparation varies widely, it is desirable that there be a feed back fromthe control pressure ied to the motor so as to introduce a temporarycompensation or throttling which will gradually reduce as the measuredvalue of the controlled quantity returns to correspondence with thedirect set operating point. The compensation in the present Fig. 1construction is effected by changes in the motor position, or perreversedirection from its initial transient movement and (2) by the change inthe indication in the sensitive measuring element I' (or in the pressureappearing in the bellows la from the sensitive element transmitter)which reiiects, or indicates the compensating' action resulting from thechange in motor position. The bellows 39 is supplied by a line 4D'connected tov a cylinder 4l. The line 40. has an adjustable restriction42 which vents to atmospherev so that under steady state conditions, thepressure in theV bellows 39 is atmospheric. Changes in the pressure inthe bellows 39 above and below atmospheric are. eirected by a lever 42ahaving one end carried on a fixed pivot 42h and the other end pivoted at42c to the valve. stem 31D. A connecting rod 43 adjustably connectedalong the length of the lever 42a transmits the desired proportion ofthe movement of the valve stem 31h to a piston 44 in the cylinder di.The movement of the piston 44 corresponding to changes in the. positionof the valve stem Sib appear in the bellows 39 as the result of movementof the piston 44 in the cylinder 4l. Initially, the change in pressurein the bellows 39 corresponds exactly to the movement of the piston 44and acts in the direction to move a lever 45 about an adjustable pivot46 slidable on the lever 45 and on a stationary guide 41. When the pivotis xed at the upper end of the lever 45, the bellows 39v has. no effect.When the pivot 46 is fixed or adjusted at some intermediate point on thelever 45 the change in pressure in the bellows. 5,9. causes an initialmovement of the apper l5 in the sense. tov oppose the movement of thefrapper by the differential lever 9. This action of the bellows 3'9 isonly present during transient changes. As the pressure in the bellows 39returns to atmospheric duev to=bleeding through the adjustablerestriction 42, the iiapper Vis returned to the position determined bythe differential lever 9. Under very slow changes the pressure in thebellows 39; is at all times atmospheric andi there is no compensatingaction introduced. From one aspect, the bellows 39 is a throttlingbellows which is reset at a rate determined by the restriction 42.Because the pressure in the bellows 39 results solely from changes inthe motor position, the throttling action is in no way dependent uponthe control pressure for operating the. motor. The throttling action ofthe bellows 39 results solely from changes in pressure in the cylinder4I atv a ratev in excess of* the:` bleed permitted by the restriction42; Thisv is different from previous arrangements in which attempts havebeen made tomodiiy thecontrol pressure and in which servo mechanism hasbeen interposed between the control pressure and the motor iny order tomore accurately position the motor. In the present arrangement, nocompensation is introduced until thc motor has moved. This permits theuse of larger initial control .pressures for operating the motor whichis convenient in the case, ofv friction loads caused. by sticky valve.stems. Furthermore, by having the. compensation. tiedintothe changes inthe Inc-tor position, therel is a reduction vof the delay time betweendeviations of measurement from the operating point and the actualcontrol action corresponding tothis deviation. The tying of thecompensation tothe changes in the motor position also eliminates thephenomenon in the nature of' hysteresis which, when present, preventsaccurate positioning of the motor since the motor position can diier forthe same control pressure in the line 3.5 depending upon whether thepressure was increasing or decreasing. From one aspect, the response ofthe air controller is directly adjusted by the piston 44 and associatedparts connected to and movable with the movable member or valve stem 31hwhich applies an actuating force through the valve 31a to the controlledquantity.

In Fig. 3 is shown a. modification especially designed for locationremote from the controller. In this modification the line tb from thesensitive element transmitter which transmits a pressure correspondingto the measured value of the controlled quantity is fed to a chamber 48between diaphragms 49 and 50, the diaphragm 49 being loaded by a spring5| and being larger than the diaphragm 50. The line Eb from the setpoint transmitter which transmits a. pressure corresponding to theoperating or set point for the controlled quantity is fed. to thechamber 5Ia between the diaphragm` 50 and the larger diaphragm 52 whichis loaded by a spring 53. The underside of the diaphragm 49 is exposedto atmospheric pressure and the upper side of the diaphragm 52 isconnected to atmospheric pressure through an adjustable restriction 55so that under steady state conditionsv when. the set point and sensitiveelement transmitted pressures are equal the upper side of thevdiaphragm. 52 is exposed to atmospheric pressure. All three diaphragms49, 5B and 52 are connected together by a center pin 55 the lower end ofwhich cooperates with a nozzle 56 corresponding to the nozzle I6 in theFig. 1 construction. The separation of the pin 55 from the nozzle, whichcorresponds to the separation of the iiapper t5 from the nozzle I6 inthe Fig. 1 construction, determines the nozzle pressure, which when fedto a control pressure relay produces a control pressure in line. 35 tothe iuid pressure motor 3l. Under steady state conditions, theseparation of the pin 55I from the nozzle 56 remains essentiallyconstant varying only slightly with the increase and decrease of thetransmitted pressures from the set point transmitter and from thesensitive element transmitter.

While the control pressure relay shown in Fig. 2 could be used in theFig. 3 system, the control Vpressure relay there illustrated isA thatshown in application Serial No. 108,775. In thisrelay the air pressuresupply il is connectedy to a way 5l leading tor a chamber 58 containinga valve 59 having oppositely facing conical faces 60 and 6i respectivelycooperating with. a seat 62 controlling the flow of pressure irom the'way 51 and a seat 63 exhausting to atmosphere. The Valve is normallybiased by ay compression spring 64 against the seat 62. Under operatingconditions the valve iioats in a positionintermediate to seats 52 and 63and thereby produces a control pressure in a way 65 leading to thecontrol pressure line 35. The valve 59 has a stem 66 extending through aslightly larger passageway 61 into a chamber 38 equivalent to thatproduced by the orifice la in the Fig. 2 control pressure relay. Sincethere is a continual back and forth or reciprocating movement of thestem 66- in the passageway 61, the. tendency to clogging of the pas--sageway is substantially eliminated. VFrom one aspect the stem 66 andthe passageway 61 provide a self-cleaning pressure reducing orificesupplying the chamber 68. A way 99 leading from the chamber 98 conductsthis reduced pressure to the nozzle 55. As in the previously describedcontrol pressure relay, the pressure in the chamber 55 is controlled bythe separation of the pin 55 from the nozzle 56. The pressure within thechamber 59 acts on an impervious diaphragm 19 which may be rubber orother suitable material. Bonded to the rubber at the center is a metalplug 1l, the upper end of which contacts the lower end of the valve stem65. Extending through the plug 1l is a ne hole 12, which provides arestricted flow from the chamber 68 into a large chamber 13 across thetop of which is a larger diaphragm 14 having a metal plug 15 bonded atits center. The upper side of the plug 'i5 has a projection 15 whichabuts a similar projection 11 on the under side of the plug 1I. Thelower side of the diaphragm 14 is closed by a cover 13 carrying anadjusting screw 19. The screw 19 is hollow and slidably carries thereinthe pin 35 which is free to move upward but has its downward movementrestricted by a pin 8| extending through the pin 89 and received inslots 52 in the adjusting screw 19. The upper end of the pin 89 bears ona cap 83 at the lower end of compression spring Bil. The spring 94normally holds the plugs and 1l in engagement and against the lower endof the valve stem 55. The amount of force exerted by the spring 84 isadjusted by the screw 19.

Under some conditions it is desirable that the valve completely shut oirthe exhaust so that the out-put pressure fed to the control pressureline can be manually controlled by varying the supply pressure l1. Thisis accomplished by pushing the adjusting pin 59 upward until the pin illis clear of the slots 82 and then turning pin through a quarter turn.This suiiioiently increases the compression of the spring 84, so thatthe valve 59 is held shut against the exhaust seat 69. This change iseiected without changing the adjustment of the adjusting screw 19.

Under steady state conditions when the pin 55 occupies a fixed positionrelative to the nozzle 56, the pressure in the chambers 68 and 13equalizes through the fine hole 12 and the position of the valve 59 isdetermined by the force exerted by the nozzle pressure on the diaphragm14. Under transient conditions, when the nozzle pressure is subject towide variations due to changes in position of the pin 55, the pressurein the chamber 5S is substantially instantaneously responsive to thechange in nozzle pressure while the pressure in the chamber 13 lagsbehind the nozzle pressure. Under transient conditions, the smalldiaphragm 19 is at rst fully efective to change the position of thevalve 59 and thereby start to alter the outlet or control pressure inaccordance with the change in the nozzle pressure. Of course, as thepressure in the chambers 68 and 13 equalize through the fine hole 12,the large diaphragm 1li comes progressively into play. If the change innozzle pressure is in the direction to increase the pressure in thechamber 68 the small diaphragm 19 moves downward with the largediaphragm 14. Ir" the change in nozzle pressure is in 'the direction todecrease the pressure in chamber 55 and the decrease is large enough,the small diaphragm may actually move away from the large diaphragm.Having the small diaphragm free to move away from the large diaphragmperphragm 99 loaded by a spring 109.

mits a quicker response which is advantageous in transient conditions.

The control pressure relay shown in Fig. 3 continues the features of thecontrol pressure relay illustrated in Fig. 1 of having a quick acting,less sensitive diaphragm supplemented by a slow acting, more sensitivediaphragm to control the position of the valve. The sensitivity of thediaphragrn can be expressed as the ratio of the change in out-putpressure to the change in nozzle pressure. The large diaphragm(exemplied in. the Fig. l relay by the bellows 26) ordinarily will havea sensitivity as measured by the ratio of change in the control pressureto the change in nozzle pressure of from 10 to 20 times the sensitivityof the small diaphragm.

The control pressure relay, just described, can of course be substitutedin the Fig. 1 control.

As in the Fig. 1 control, the control pressure acts on the fluidpressure motor 31 in the direction to bring the controlled quantity backto the operating or set point. Where the fluid pressure motor controls avalve, the position of the motor will be represented by the valve stem311).

The arrangement for feeding back the move ment of the valve stem 31ocomprises a bracket 55 iixed to the valve stem and connected by a link35 to a lever 81 pivoted at 89. The lever 9'! carries a valve 89 whichcooperates with a diaphragm 99 loaded by a spring 9|. The diaphragm 90forms the upper wall of a chamber 92 fed from a compressed air line 93through a restriction 93a. With this arrangement, the pressure in thechamber 92 corresponds to the position of the valve stem 31h. Uponchanges in position or" the valve stem, the valve 99 changes the rate ofbleed and thereby causes a movement of the diaphragm until the pressurebalances the spring 9i. The pressure from the chamber 92 iiows through away 94 and is divided in a pron portion determined by an adjustablevalve 95 between a way 96 leading to a chamber 91 and a way 98 leadingto the atmosphere. The chamber 91 is on the upper side of a flexibledia- The diaphragm 99 corresponds to the piston 44 in the Fig. 1construction. Upon sudden movement of the valve stem 31h thereaccordingly is a sudden change in the pressure in the chamber 92 whichis fed through the ways 94 and 96 and causes a corresponding suddenchange in pressure in the chamber 91 on the upper side of the diaphragm99. The lower side of the diaphragm 99 forms the upper wall of a chamberlill which is connected through a way |02 to a chamber |93 on the upperside of the diaphragm 52. The way 192 is connected through a passageway194 which bleeds to atmospheric pressure through the restriction 5d.Upon a sudden movement of the diaphragm 99 either up or down there isproduced in the chamber I 03 a pressure respectively below and aboveatmospheric which lasts for a length of time depending upon therestriction 54. Since under steady state conditions, when the pressurein the chamber |03 is atmospheric the pin 55 has the proper separationfrom the nozzle 56 so as to produce a control pressure exactlycorresponding to the operating point of the controlled quantity, it isevident that raising or lowering the pressure in chamber |03 willproduce a substantial movement of the pin 55. This movement is in thedirection to initially compensate for the movement of the valve stem3119, but the amount of compensation is gradually reduced by bleedingthrough the restriction 54 so 9 that as the valve stem 31h reaches thecorrect position for steady state operation the compensa tion isentirely removed.

For example, if the controlled quantity should suddenly drop below theoperating point as determined by the set point transmittere there wouldbe a corresponding reduction in the pressure in line lb from thesensitive element transmitter and a corresponding upward movement of thepin 55 due to the reduction in pressure in the upper side of thediaphragm V49, there being no change in the pressure on the diaphragms55 and 52. The separation of the pin 55 from the nozzle 55 caused by its-upward movement results in a decrease in nozzle pressure whichdecreases the pressure in the chamber 68 and accordinalv causes anupward movement of the valve 59 resulting in. an increase in the controlpressure in the line 35 due to the restriction of the bleed through theseat 63. in the line 35 causes a downward movement of the valve stem 31hto open the valve 31a Wider and to increase the value of the controlledquantity. Until there is an actual movement of the valve stern 31h,there is .nothing to change the position of the pin 55. Upon downwardmovement of the valve stem 31h there is an upward movement of the valve89 which raises the pressure in the chamber 92 and causes an increasedpressure to be transmitted through the ways 913 and 96 to the upper sideof the diaphragm el. The increased pressure causes the diaphragm 9T tocollapse and produces a greater than atmospheric pressure on the upperside of the diaphragm 52. This greater than atmospheric pressure on theupper side of the diaphragm 52 compensates for the decrease in pressureon the upper side of the diaphragm 49 and accordingly tends to move thepin 55 back toward the nozzle 56. As the pressure in the chambers l! and|03 bleeds to atmosphere through the restriction 54. the compensation orthrottling action due to above atmospheric pressure on the upper side ofthe diaphragm 52 is removed and the pin 55 is permitted to return or tobe reset to the position determined by the difference between the setpoint and sensitive velement transmitter pressures. Since the bleed toatmosphere through the restriction 54 Vis related to the time requiredfor the valve 31a to restore the controlled quani tity to the operatingpoint, the removal of the compensating action caused by the bleeding oirdoes not take place until the controlled quantity.

has returned to the operating point.

The Fig. 3 control continues the advantage of the Fig. l control ofhaving the motor control pressure dependent upon the deviation of thecontrolled quantity from the set or operating point and of having thecompensation or throttling action decreasing the sensitivity or respceeof the control dependent upon the movement of the fluid pressure motoroperated by the icontrol pressure. By tying the sensitivity compensationto the motor position,the eiect of hysteresis is eliminated.Furthermore, it is possible to operate the control on its high gainrange for a longer time and thereby obtain a quicker actual correctionfor deviations in the controlled quantity'from the operating point.

In all of the controls, there is a multiple feed back to the controllernozzle and baille (l5, it Fig. 1; 55, 56 Fig. 3). Changes in measurementof the Vcontrolled quantity are fed back by the The increase in pressurel() measuring instrument I or through the line ib from the measurementtransmitter.

There obviously is some delay in measuring and in receiving changes inmeasurement. The other feed back is from changes in the motor position(slib) which is much quicker than the feed back from measurement. Thefeed back from changes in motor position, whiie initially much fasterthan the feed back from measurement, is reset to zero at a rate which issubstantially equal to or less than the response time for the measuringinstrument and its transmitter. When the change in the controlledquantity has been rei'iected by measurement, the feed back from thechange in motor position has been completely or at least partly reset tozero. This prevents instability or hunting. If the reset time for thefeed back from changes in motor position were shorter than the responsetime for measurement of changes in the controlled quantity there wouldbe a tendency toward instability. f

The term bellows is used to include diaphragms.

What I claim as new is:

l. In a control, a transmitter producing a pressure corresponding to themeasured value of a controlled quantity, a transmitter producing apressure corresponding to the set or operating value of the controlledquantity, a controller for producing a Control pressure corresponding tothe diiierence between the transmitted pressures, a motor fed from thecontrol pressure actuating an element vai ing the controlled quantity, apneumatic means for changing the sensitivity of the controller, meansactuated bychanges in the motor position for producing a` correspondingchange in pressure, means instantaneously feedthe change in pressure tothe pneumatic in the sense to decrease the sensitivity of thecontroller, and time delay means gradually disabling the sensitivityreduction of the contrailer.

2. In -a control, a controller for producing a control output pressurecorresponding to the difference between the measured and set oroperating vaiues of a controlled quantity, a motor fed from the controloutput pressure actuating an element varying the controlled quantity, apneumatic means for changing the sensitivity of the controller, meansactuated by changes in the motor position for producing a correspond-Ving change in pressure, means instantaneously chamber, means actuatedby changes in the.

motor position for producing a corresponding change in the volume of thechamber with a resultant change in pressure in the chamber, meansinstantaneously feeding the change in pressure in the chamber tothebellows in the sense to decrease the sensitivity of the controller, anda restricted bleed from the chamber gradually disabling the sensitivityreduction of the controller due to changes in the pressure in theexpansible chamber with itsl changes in volume.

4. In a control, a controller for producing a control output pressurecorresponding to the difference between the measured and set oroperating values or" a controlled quantity, a motor fed from the controloutput pressure actuating an element varying the controlled quantity, apneumatic means for changing the sensitivity of the controller, meansactuated by changes in the motor position for producing a correspondingchange in pressure, means instantaneously feeding back to the pneumaticmeans, in the sense to decrease the sensitivity of the controller, thechange in pressure corresponding to the changes in motor position, andtime delay means gradual- 1y counteracting the feed back of the changesin pressure corresponding to the changes in motor position.

5. In a control, a controller for producing a control output pressurecorresponding to the difference between the measured and set oroperating values of a controlled quantity, a motor fed from the controlpressure actuating an element varying the controlled quantity, means forchanging the sensitivity of the controller, means actuated by changes inthe motor position for producing a control impulse, meansinstantaneously feeding the control impulse to the sensitivity changingmeans in the sense to decrease the sensitivity of the controller, andtime delay means gradually disabling the control impulse,

References cited m the nl@ of this patent UNITED STATEs PATENTs NumberName A Date 2,303,752 Meredith Dec. l, lil/l2 2,372,345 Temple a Mar.27, 1945 2,409,871 Krogh Oct. 22, 194.5 2,484,557 Eckman Oct. 11, 19492,487,266 Newell Nov. 8, 1941.9 2,516,333 Moore July 25, 1950

